The present invention relates to a developer solution for a positive-working resist composition. More particularly, the invention relates to a developer solution for a positive-working resist composition of the type comprising an alkali-soluble novolac resin as the film-forming resinous component and a naphthoquinone diazide compound as the sensitive component and capable of forming a resist pattern by irradiation with actinic rays. In particular, the invention has an object to provide a developer solution capable of greatly improving the contrast and depth of focus of patterned images formed from a positive-working resist composition.
As is well known, manufacturing processes of various kinds of electronic devices and components such as semiconductor-based integrated circuits, photomasks for the manufacture of integrated circuits, printed circuit boards and the like always involve a step of an area-selective surface treatment of the substrate such as etching treatment, diffusion treatment and the like. It is usual in such a selective surface treatment that a patternwise layer of a resinous resist composition having sensitivity to actinic rays such as ultraviolet light, X-rays, electron beams and the like is formed on the substrate surface with an object to protect the areas of the substrate surface which should not be subjected to the surface treatment.
Such a patternwise protecting resist layer is formed on the substrate surface by a method including the steps of forming a coating layer of a resist composition on the substrate surface, irradiating the resist layer patternwise with actinic rays and developing the latent images formed by the irradiation with a developer solution. Resist compositions are classified depending on the types of sensitivity to the actinic rays into positive-working ones, of which the areas of the resist layer irradiated with actinic rays are imparted with increased solubility in developer solutions while the unirradiated areas are left insoluble, and negative-working ones, of which the solubility behavior of the irradiated and unirradiated areas in developer solutions is reverse to that in the positive-working resist compositions.
Along with the trend in recent years toward higher and higher dimensional fineness of working in the manufacture of electronic parts in general, prevalence in use is directed to the positive-working resist compositions over negative-working ones owing to the advantage of the high resolving power of the former as compared with the latter. Positive-working resist compositions of a typical type are formulated with an alkali-soluble novolac resin as the resinous film-forming ingredient and a naphthoquinone diazide compound as the ingredient having sensitivity to actinic rays. The resist layer formed from such a positive-working resist composition formulated with a novolac resin and a naphthoquinone diazide compound can be developed with an aqueous alkaline solution as the developer solution, in which the water-soluble alkaline compound is usually an organic alkaline compound such as tetramethyl ammonium hydroxide, choline, i.e. trimethyl 2-hydroxyethyl ammonium hydroxide, and the like. Though useful in most cases as a developer solution for positive-working resist compositions, one of the problems in the developer solutions of this type is that the difference in the solubility behavior is relatively small between the areas unirradiated and irradiated with actinic rays or the contrast in the image formation so that the cross sectional profile of a line pattern of the developed resist layer sometimes has trailing skirts on the substrate surface to greatly decrease the accuracy or fidelity of the resist pattern. This problem is particularly serious when the solution is used for the development of a resist pattern having increased fineness reaching the so-called submicron order. Accordingly, the performance of the developer solution available is a limiting factor which prohibits compliance with the demand for an increased dimensional fineness of working in the manufacture of modern semiconductor integrated circuits.
On the other hand, improvements in the technology of irradiation with actinic rays also have a great influence to facilitate an increase in the working fineness in the manufacture of semiconductor devices. For example, patterned resist layers having a submicron order fineness can easily be obtained by the techniques of minifying projection exposure recently developed utilizing a lamp emitting the i-line ultraviolet light having a short wavelength of 365 nm as the light source. Nonetheless, this method cannot be without any problems in the practical application in respect of the problem relative to the depth of focus.
This problem in the depth of focus or the problem that the depth of focus is decreased when the resolving power is increased is unavoidable in so far as the irradiation apparatus for the patternwise irradiation utilizes an optical system. However, the disadvantage due to a decreased depth of focus can be compensated for by using a developer solution capable of giving a high-contrast image by which an effective resist pattern can be obtained even with a small depth of focus and samll width of light-intensity distribution.
In this regard, various proposals have been made for the formulation of a developer solution for positive-working resist compositions with an object to increase the contrast of the developed images including an aqueous solution of a cyclic amine compound as a developer solution for a 1,2-naphthoquinone diazide-based positive-working resist composition disclosed in Japanese Patent Kokai No. 57-30832, an aqueous soluion of a cyclic quaternary ammonium hydroxide compound disclosed in Japanese Patent Kokai No. 60-179738 and the like. The developer solutions of the former type are defective because, although they are useful for the development of a positive-working resist composition of which the resinous ingredient is a copolymer of an .alpha.,.beta.-unsaturated olefinic carboxylic acid, they are not applicable to the positive-working resist compositions comprising a novolac resin as the film-forming ingredient used in the major current of the modern technology. The developer solution of the latter type is also disadvantageous and not practicable because the cyclic quaternary ammonium hydroxide compound as the principal ingredient of the aqueous solution must be prepared in a very complicate process including a step of N-methylolation of a cyclic nitrogen compound followed by ion exchange so that stability in the developing performance of the developer solution can hardly be expected as a consequence of the complicate preparation procedure of the solution not to give reproducible results in the patterning of resist layers.